Bonded high energy rare earth permanent magnets

Abstract

A permanently magnetizable composite is made from rare earth permanent magnet material by incorporating coarse particles of the material into a workable binder at a gradual rate such that sparking and combustion are avoided, until the particles are coated with or embedded in the binder, and then working the mixture under shearing forces sufficient to break up the particles in situ, thereby forming much finer particles directly within the binder. Extremely high particle packing fractions can be obtained in this manner, yet the residual induction of even an isotropic composite substantially exceeds the maximum that would be expected for a magnet having that packing fraction.

Having described the invention, what is claimed is: 
     
       1. A process for making bonded magnets of the rare earth type having unexpectedly high magnetic properties, comprising: adding particles of rare earth type permanent magnet material to a workable non-magnetic binder until the particles are coated with and cohered in the binder as a workable mass; the average size of the particles of the starting magnet material being substantially larger than single domain size for said magnet material, less than about 15 weight % of the particles of said starting material being smaller than 44 microns in size;   thereafter working said mass under shearing forces of such intensity that the average size of the particles is substantially reduced by such working, thereby improving the magnetic properties of the particles,   progressively reducing the thickness of said mass during said working while simultaneously increasing its area to form an extended shape therefrom,   forming desired magnet shapes from said sheet, and   magnetizing said magnet shapes to form permanent magnets therefrom,   said magnets having a residual induction which exceeds by at least 5% the maximum value expected for their packing fraction.   
     
     
       2. The process of claim 1 wherein the particles of the starting magnet material are isotropic, and wherein the product is similarly isotropic. 
     
     
       3. The process of claim 1 further wherein the average size of the particles in said mass is reduced by at least 50% during said working. 
     
     
       4. The process of claim 1 wherein particles of said magnet material are added to said binder until the packing fraction of magnet particles in said mass is at least about 0.65. 
     
     
       5. The process of claim 1 wherein said magnet material is a rapidly quenched neodymium-iron-boron permanent magnet alloy. 
     
     
       6. The process of claim 1 wherein said magnets have a maximum energy product of at least 6.0 GaussOerstead. 
     
     
       7. The process of claim 1 wherein said binder is a copolymer of ethylene and vinyl acetate. 
     
     
       8. The process of claim 1 wherein said particles are magnetized before they are added to said binder, thereby minimizing suspension of such particles in air. 
     
     
       9. The process of claim 1 wherein said working is carried out in a screw-type mixer. 
     
     
       10. The process of claim 1 wherein said working is carried out in a two-roll mill. 
     
     
       11. The process of claim 1 wherein the packing fraction is at least 0.65; at least about 85% wt. % of said particles being in the range of 44-420 microns and the residual induction of said permanent magnets is at least 5000 Gauss. 
     
     
       12. The process of claim 11 wherein said mixture is worked into sheets which are then layered and reduced in size, at least nine times. 
     
     
       13. A durable, bonded, edge cuttable isotopic composite permanent magnet comprising particles of permanent magnet material incorporated in a workable binder, said magnet having a measured residual induction B rc  which is at least 5% greater than the amount calculated for that magnet from the expression, ##EQU10## where P f  is the particle packing fraction of the composite magnet; B r  is the measured residual induction of the magnet material;   S is the shope B r  /H c  of the starting material; and   H c  is the measured coercity of the magent material,   the average size of the particles having been substantially reduced in situ by shear forces exerted on them through the working of coarser starting magnetic particles into the binder.   
     
     
       14. The magnet of claim 13 wherein the residual induction is at least 10% greater than would be predicted from its packing fraction. 
     
     
       15. The magnet of claim 13 having a packing fraction of at least about 0.60. 
     
     
       16. The magnet of claim 13 wherein said permanent magnet material is a rare earth magnet material. 
     
     
       17. The magnet of claim 17 wherein said permanent magnet material is rapidly quenched neodymium-iron-boron. 
     
     
       18. The magnet of claim 18 wherein said particles are of a size so small as to be pyrophoric if not embedded in said binder. 
     
     
       19. The magnet of claim 13 wherein said binder is a copolymer of ethylene and vinyl acetate. 
     
     
       20. The process of claim 1 wherein said binder is a silicone rubber. 
     
     
       21. The magnet of claim 13 wherein said binder is a silicone rubber. 
     
     
       22. The process of claim 1 wherein said binder is a polymer which is chemically compatible with the particles of said magnet material. 
     
     
       23. The magnet of claim 13 wherein said binder is a polymer which is chemically compatible with the particles of said magnet material.